Technical Abstract:
Efforts have been under way to enhance plant iron content because of the need to increase dietary iron levels for over one-third of the world's population. Although root iron uptake is an important determinant of plant iron content, a number of subsequent chelation, transport, signalling, and storage processes ultimately regulate the iron content of edible components, such as seeds. Taken together, these processes constitute the homeostatic mechanisms used to ensure that sufficient iron is available for plant growth, while at the same time preventing overaccumulation of this potentially toxic metal. We have used two, iron-hyperaccumulating mutants of pea to understand iron homeostasis. Both the brz and the dgl mutants exhibit elevated rates of root Fe(III) reductase activity, which can lead to toxic iron levels (>100-fold wildtype). brz and dgl are not alleles of the root reductase gene; rather, they appear to encode proteins involved in nor which disrupt the plant's ability to sense iron status in the shoots. Reciprocal shoot-root grafting studies have shown that both mutations lead to the expression of a phloem-mobile signal molecule, originating in shoots, which upregulates root iron acquisition processes. With respect to long-distance transport, studies with these mutants also have revealed that phloem iron loading and transport requires iron chelation. Merely increasing iron delivery to vegetative tissues is not sufficient to alter seed iron content, as the iron levels of both brz and dgl leaves can be increased 30-fold, but only dgl seeds show elevated iron content (4-fold relative to wildtype). Identifying these chelators, along with the phloem- mobile signal molecule, will allow us to manipulate the plant's homeostatic controls, and thereby enhance the iron nutritional quality of plant foods.